One of the big problems that confronts metallic objects in certain environments is the corrosion of these objects. Various rust preventatives have been developed over the years from combinations of metals to rust preventive paints and coatings.
However, one of the most effecive methods of corrosion protection is known as cathodic protection. Such a method is particularly applicable to metallic pipelines, underground metallic tanks, structures in contact with the earth, metallic ship hulls immersed in water, etc. Depending on polarity this method affords either cathodic or anodic protection. Anodic polarization is used in some specialized instances.
Cathodic protection may be achieved by making a metallic structure the cathode in an electrolytic cell. The anode in this cell may be of a metal more active, i.e. more positive in the electrochemical series, in which case, consumption of the anode furnishes the power required for the process. The more commonly used metals used in conjunction with steel or iron are, aluminum, magnesium, and zinc. Each of these metals has its particular adaptability to a situation. These anodes are often bonded, bolted or otherwise connected through a metallic circuit to the structure to be protected.
Other metals or substances less reactive such as iron, lead, and platinum may also be used as anode material. In this case, a power supply will be required to establish a flow of electricity in the system.
The current established in the electrical circuit from anode to the soil (electrolyte), soil to cathode, through the metallic path back to the anode will polarize the electrodes at the electrolyte interface. Polarization being the formation of an electromotive force (EMF) on the surface of the electrode in opposition to the established current. The degree of polarization is a measure of the level of protection of the subject structure or cathode.
With the development of the cathodic system of protection, it has also been desirable to be able to measure the polarization potential of the protected structure.
To accomplish this, suitable reference electrodes, or half-cells, are used with an electrometer, such as a conventional voltmeter or millivolt meter will be able to afford visual readings of the volt or millivolt polarization potential. Based upon a range of volts or millivolts required for the particular structure, readings can be taken and a determination made as to whether there is adequate cathodic protection of the metal structure.
There is, however, a difficulty with this mode of measurement because with a constant current the voltmeter in reality records the sum of two voltages, that sought and the other that is generated by the passage to the structure by the electric current in the environment between the reference cell and the structure.
One way in which efforts have been made to produce a true reading instead of a mixed reading is that of separating these two voltages by interrupting the current momentarily so that the potential may be observed when the current is off. This is usually done by cutting off the current for a preselected arbitrary time period which usually runs for 5 to 10 seconds, whereon at the conclusion of the period current is then turned on for a period of time, usually 25 seconds to 50 seconds. Such a schedule or "repeat cycle" is conveniently accomplished by an appropriate timer mechanism.
However, while it has been found that the "repeat cycle" technique is valuable for achieving a truer reading of the polarization potential of the metallic structure to be protected, other complications arise.
First the meter indicates a value which is the sum of the two voltages. Secondly, on interruption of the current, a new lesser value is instantaneously "felt at the meter," then while the current is "off" there is a rapid decay of the polarization potential which is reflected in movement of the needle of a voltmeter. It has been determined that such decay is a logarithmic function of several combined factors. Such factors include the degree of polarization at the particular time, the resistivity of the enviornment, and the effects of adjacent portions of the metallic structure being protected. The greater the degree of polarization, the faster the decay. The instantaneous change in voltage reflecting the electrical phenomena, at the instant of "off", cannot be read on a conventional voltmeter.
Most voltmeters of a sufficient sensitivity or accuracy cannot respond rapidly enough to indicate the fleeting or transient instantaneous "off" potential. Interpretation of data so obtained may result in assuming that at least some portions of a cathodic protection system is in jeopardy, when in fact this condition did not exist. In other words, it is impossible with present voltmeters to observe an instantaneous change from a high reading to a lower reading.